Position servo systems are known to be utilized for accurate positioning of a moveable member in response to a position request signal. These systems generally include an actuator for providing a force to move the member to a desired position, as well as a position transducer for determining the actual position of the moveable member. Known position transducers include for example potentiometers, variable inductances or capacitances, resolvers, magnetic or optical encoders, differential transformers and others. An error signal is determined as a difference between the position request and actual position signals and is applied through appropriate circuitry to a servo motor whose moveable portion is attached to the member. In applications with strict requirements related to positioning accuracy it is desirable to reduce the steady state position error to minimum and ideally to zero.
It is generally known to mount the moveable member on a spring providing a force opposing the motion of the member from an initial position. Voice coil motors, such as voice coil type audio frequency speakers, are an example of known actuators. These actuators require a small amount of position error to overcome the force of the spring while keeping it deflected in the desired position. This introduces a steady state position error into the servo loop. Efforts to reduce the error by increasing the loop gain have limited success because of the need to avoid exciting system resonances. While it is possible to compensate for the opposing force of the spring by integrating the error signal, this method introduces a phase lag into the feedback loop, thereby increasing servo instability and complicating the stabilization requirements.
An example of a known servo system for positioning a moveable magnetic transducer utilized in a magnetic recording/reproducing device is described, for example in the U.S. Pat. No. 4,485,414. An example of a leaf spring assembly for holding and varying the position of a magnetic transducer is described in U.S. Pat. No. 4,212,043. Both above-indicated patents are assigned to the assignee of this patent application.